Problem: 10 markers cost $10.00. Which equation would help determine the cost of 7 markers?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 10 markers. We want to know the cost of 7 markers. We can write the numbers of markers as a proportion: $\dfrac{10}{7}$ We know 10 markers costs $10.00. We can let $x$ represent the unknown cost of 7 markers. The proportion of these costs can be expressed as: $\dfrac{\$10.00}{x}$ The cost changes along with the number of markers purchased, and so the two proportions are equivalent. $\dfrac{10}{7} = \dfrac{\$10.00}{x}$